A large portion of the chlorine and alkali metal hydroxide produced throughout the world is manufactured in diaphragm-type electrolytic cells wherein the opposed anode and cathode are separated by a fluid permeable diaphragm, usually of asbestos, defining separate anode and cathode compartments. In a typical operation, saturated brine is fed to the anode compartment wherein chlorine is generated at the anode, the brine percolating through the diaphragm into the cathode compartment wherein sodium hydroxide is produced at a concentration within the range of 11 percent to 18 percent and "contaminated" with large amounts of sodium chloride. The sodium hydroxide must then be concentrated by evaporation and the chloride must be removed to provide a commercial product.
Through the years, substitution of a membrane material for the diaphragm has been proposed. These membranes are substantially impervious to hydraulic flow. In operation, an alkali metal chloride solution is again introduced into the anode compartment wherein chlorine is liberated. Then, in the case of a cation permselective membrane, alkali metal ions are transported across the membrane into the cathode compartment. The concentration of the relatively pure alkali metal hydroxide produced in the cathode compartment is determined by the amount of water added to this compartment, generally from a source exterior the cell. While operation of a membrane cell has many theoretical advantages, its commercial application to the production, for example, of chlorine and caustic has been hindered owing to the low current efficiencies obtained and the often erratic operating characteristics of the cells.
More recently, much improved membranes have been developed to overcome many of the prior problems. The most promising such membrane is a thin film of fluorinated copolymer having pendant sulfonyl fluoride groups thereon such as described in U.S. Pat. Nos. 3,041,317; 3,282,875; and 3,624,053 and the like. Such membranes in hydrolyzed form are available from E. I. duPont de Nemours and Co. under the trademark NAFION.
These membranes can be further improved by surface treatments which consist of reacting the sulfonyl fluoride pendant groups with ammonia gas or more preferably with an amine which will yield less polar binding and thereby absorb fewer water molecules by hydrogen bonding such as described in detail in applicants' copending applications Ser. Nos. 587,047 and 686,179. The more efficient of these modified membranes are highly cross-linked and become extremely brittle especially in commercial dimension.
To further improve on these modified membranes a fabric reinforcing material has been laminated to such membranes by the application of heat and pressure. Such treatment, however, has resulted in improvement from a mechanical standpoint but is found lacking in that the heat and pressure required in the fabric bonding operation impairs if not completely destroys the effectiveness of the amine modified surface.